It is well known that two thermoplastic films may be extruded and laminated together forming trapped air pockets or cells. For example, U.S. Pat. No. 3,142,599 to Chavannes and assigned to the assignee of the present invention, discloses such laminated cellular cushioning material. A first extruded film is brought into contact with an embossing roll typically having a pattern of cavities and lands formed in the outer surface thereof. To enhance conformance of the plastic film with the cavities, each cavity typically includes a opening through which suction may be applied to draw the plastic material down into the cavity as shown, for example, in U.S. Pat. No. 3,208,898 also assigned to the assignee of the present invention.
A second film is then extruded and brought into contact with the first film. The two films fuse ether adjacent the land portions of the embossing roll trapping pockets of air therebetween. The cellular laminate thus formed is widely used as cushioning material for packaging products for protection during shipping and storage.
To ensure adequate mechanical contact between the first and second plastic films where they are brought into mutual contact on the embossing roll, a nip roll assembly is commonly used. The nip roll assembly includes a nip roll positioned opposite the embossing roll so as to physically contact and press the two films together on the embossing roll. The plastic films have a relatively high temperature at the contact point. Accordingly, the nip roll must contact the relatively hot plastic film yet release the film upon further rotation. Typically, the nip roll has a Teflon covering so that the hot plastic film does not adhere to the roll and result in continuous wrapping of the film around the roll. Unfortunately, the service life of the nip roll and Teflon covering is relatively short. In addition, the nip roll assembly must typically include an auxiliary cooling roll for cooling the surface of the Teflon and to minimize deflection of the nip roll against the embossing roll.
Any repair or recovering of the Teflon surface of the nip roll necessitates manufacturing production downtime while the roll is being changed out. In addition, such repair or replacement is often relatively expensive. For example, because of the unique design of the Teflon covered nip roll, the replacement cost may exceed several thousands of dollars per roll. Durability of a repaired or recovered nip roll is unclear at best. The range is typically somewhere between one day and three months. Temporary short term repairs are often done, but can adversely affect the cellular product. To simply replace the cover is also relatively expensive, even excluding the cost of production downtime.
The opposing nip roll often does not provide consistent quality--leaving gaps in the lamination so that air transfers between adjacent cells or bubbles, for example. The embossing rolls are also typically formed of several cylindrical sections which are positioned in end-to-end relation. Unfortunately, adjacent sections may not be in perfect alignment. Any imperfection in the alignment at the joints between adjacent sections is also likely to yield inconsistent lamination. Heavy gauge extrudate may mask any irregularities at the joints between adjacent sections; however, for light gauge extrudate, the irregularities may be exaggerated, thereby producing an unacceptable product. Cell-to-cell air transfer may be evident along the portion of the laminate formed at the section joints, thereby reducing the cushioning effect of the cellular laminate. Softer roll coverings and other materials have been tried on the nip roll, but all have failed as a result of the extreme heat from the plastic films.
In other plastic film production processes it is known to impress an electrostatic charge onto a single extruded plastic film to cause the film to be drawn into contact with an electrically grounded cooling, or quenching, roll. For example, U.S. Pat. No. 4,111,625 to Remmington et al., discloses an apparatus for applying an electrostatic charge to a single molten plastic film to "pin" the film to a rotating quenching drum. The electrostatic charge is applied by a wire electrode extruding transversely across the single molten film and parallel to the axis of the quenching drum. Similarly, U.S. Pat. No. 4,594,203 to Hagiwara et al. discloses a system in which the contact area of the single film and the quenching roll is maintained in a gas atmosphere having a higher electrical resistance than air to thereby prevent premature discharge of the electrostatic charge from the film to the quenching roll. Unfortunately, these pinning systems do not address lamination of two or more plastic films. Rather, they disclose that an electrostatic charge must first be applied to the single film, then the charged single film drawn into contact with the grounded metal quenching roll by electrostatic attraction.
It is known in the related field of forming plastic laminates that an electrical corona discharge may be used to treat chemically the surface of two or more films to assist in laminating the films together. The corona discharge treatment causes oxidation and other changes to the surface of the plastic film. U.S. Pat. No. 4,778,557, for example, discloses a laminating apparatus using such a corona discharge treatment of plastic films. It is preferred, however, that plastic cellular laminates are formed by heat fusion of the two plastic films together. Heat fusion forms a strong and secure bond between the two films of the laminate as required to prevent cell-to-cell transfer of air to provide consistent cushioning properties.
Despite continuing efforts to improve the performance of a conventional Teflon covered nip roll and to extend its useful service life, the nip roll is expensive to purchase and maintain and still frequently causes quality problems, such as cell-to-cell transfer of air. The electrostatic charge pinning of a single sheet to a quenching roll, or corona treatment of two films, do not address lamination of two films to form cellular laminates.